I had the same problem, including the deteriorating performance when it used to be reliable (I don't know if the timing is the same but sounds about right), and

I had the same problem, including the deteriorating performance when it used to be reliable (I don't know if the timing is the same but sounds about right), and the router fixed it for me as well. Maybe there were protocol / firmware changes that increased the demand on the router? I went from a bargain router to a nicer mesh setup this spring and the problem just vanished overnight.